In recent years, in devices such as a light coupling element and a light data link, the light emitting efficiency of the light emitting element has improved, and transmission loss in the plastic fiber which forms the light transmission line has also decreased. For this reason, by using such a device, it is possible to perform a high capacity and low cost light data communication. In such a light data communication environment, a dynamic range of a light receiving circuit could be improved though it would still be necessary to maintain a stable operation in this wider operation range.
The strength of the light signal to be transmitted in these devices varies in a range from a very low level to a very high level due to variations in the transmission distance of a light transmission line, and thus expansion of a dynamic range of a light receiving circuit within these devices becomes a significant issue. For example, in a light receiving circuit that includes a light receiving element and a trans-impedance amplifier (TIA) at an input stage, a very weak signal can be received and accommodated by setting the gain of the trans-impedance amplifier to a high level. However, if the gain is set too high, a strong signal which may also be received by the light receiving element could cause the trans-impedance amplifier to be saturated, whereby a waveform of an output signal from the trans-impedance amplifier is distorted. For this reason, an incorrect output signal from the trans-impedance amplifier can be generated, and thus, the potential incoming signal strength has to be decreased in such a device.